Magnetic resonance systems acquire data using strong magnets for providing large static magnetic fields. Gradient coils within the magnets are provided to "focus" the magnetic fields. The large static magnetic fields are used to magnetically align certain nuclei ("spins") of the sample being imaged or spectroscopically studied. A radio frequency (RF) pulse is used to "tip" the aligned spins so at least a projection of the tipped spins is in a plane orthogonal to the plane in which the spins are aligned. When the RF pulse terminates the nutated or tipped spins tend to dephase and also tend to return to the aligned condition. The movement of the spins in the orthogonal plane generate what are known as "free induction decay" (FID) signals. It is the FID signals in one form or another that are used for imaging and/or spectroscopic purposes.
While many types of magnets can be used to generate the large static magnetic fields: in a preferred embodiment a super-conducting magnet is used. The subject or patient is placed in the bore of the super-conducting magnet for exposure to the large static magnetic field.
Radio frequency coils or probes are used for transmitting RF pulses and/or receiving the FID signals. These probes are energized in a transmitting state with an RF pulse frequency known as the Larmor frequency which, as is well known, is a function of the particular element and the strength of the magnetic field in which the element is located. The Larmor frequency is also the precessional angular frequency of the aligned nuclei (spins) and the frequency of the FID signals.
The RF probes are either body probes wound around the base of the large magnet or special probes often used in addition to the body probes. The special probes are designed to be juxtaposed to particular portions of the body such as spine, limbs or the head. The probes must be capable of:
resonating at the desired radio frequency;
generating homogeneous magnetic field when the probe is used in the transmitting mode, and
adding only minimal noise to the signals received.
Surface coils are such special probes designed to be juxtaposed to particular portions of the body. Surface coils are relatively efficient due to the proximity of the probe to the body part from which data is acquired.
Notwithstanding the relative high efficiency of the proximate probes including surface coils; the signal-to-noise ratio (SNR) of the acquired data remains critical because of the small amplitudes of the FID signals. The SNR decreases because, among other things, "pick up" of stray signals by the probe caused by stray capacitances and/or mutual inductance between the coils in quadrature surface coil arrangements or in surface coil arrays. SNR is also decreased because of variations in the impedances of the probes when "loaded" by the patient. Different patients have different body impedences and, therefore, load the RF probes differently. Also, the human body is not symmetrical--thus, loading is not symmetrical and asymmetrical loading results in variations in the signals received from the probe. Signal-to-noise ratio is adversely affected by the size of the surface coil; so that when other things are equal the larger the surface coil, the smaller the signal-to-noise ratio.
Another serious problem faced by scientists and designers of MR systems is that the RF power transmitted by the probes may cause heating of the body sections being studied. The heating occurs because only a relatively small portion of the RF power tips the spins; most of the power generates eddy and dialectric currents in the tissue of the subject which in turn generate heat. This RF heating has caused the Federal Drug Administration (FDA) in the United States to set a limit on the specific power absorption rate (SAR) of the RF signal that can be used in imaging humans. The set limit is 0.4 Watts per kilogram. Thus, there is a limit on the power that can be used by RF probes. This limit is a fraction of the patient's weight. The limit is designed to safeguard the patient from exposure to RF caused heat damage to tissues.
Most probes used in the past have been linearly polarized. For example, "saddle" shaped coils have been extensively used. Linear polarized as used herein means that the field provided by the probes are normal and remain normal to one of the planes defined by two of the orthogonal axes of the MR system, generally speaking the MR systems are considered as XYZ orthogonal systems with the large static magnetic field in the Z direction. The spins precess around the Z axis, for example, and the effect of a projection of linear polarization is in the XY plane.
When using the linear polarization of the applied RF pulses, only half of the RF pulse power of the generated magnetic lines pass through the subject. Accordingly, only half of the RF power is effectively used, at best, to tip the spins. Another problem is that the presently available RF probes, including surface coils, cause what are known as radio frequency penetration artifacts which appear on the body images as shaded areas. The artifacts result from standing waves of the RF radiation passing through the tissue at high frequencies which distort the uniformity of the applied radio frequency magnetic field. In an attempt to overcome this problem, the prior art implemented an excitation mode wherein the polarization is circular rather than linear. See, for example, the Patent Application entitled "Quadrature Surface Coil" filed in the United States on Mar. 10, 1989, which received Ser. No. 321,885 and the references cited in that Patent Application. The invention of that Patent Application was invented by the inventor of the present Application and is assigned to the Assignee of the present Application.
The circular polarization in addition to improving image quality reduces the power required to achieve the given shift of the spins. The circular polarization decreases the necessary RF power by a factor of two. Accordingly, smaller RF power amplifiers can be used.
Also, less energy is absorbed by the patient; thereby reducing the problem of possibly exceeding the 0.4 Watts per kilogram SAR. The sensitivity of the receiver coils to the FID signals are also greater with circular polarization by an amount that increases the signal-to-noise ratio by a factor of the square root of 2.
A problem occurring when using quadrature mode equipment has been the difficulty in providing coils which can generate circularly polarized RF fields without being unduly influenced by the patient loading. Also, quadrature mode generating equipment is generally unduly influenced by the cross-coupling; i.e., the mutual inductance between multiple coils that must be used to generate the circular polarization. Therefore, a particularly aggravating problem is the minimization of the cross-coupling between the multiple coils.
The prior art attempts at accomplishing circular polarization or quadrature excitation have been accomplished using multiple spaced-apart coils. The multiple spaced-apart coils comprise, for example, either two coils physically at 90.degree. to each other, counter rotating quadrature current resonators, planar pair resonators or extremely complicated quadrature surface coils.
Another problem related to the use of quadrature surface coils is that none of the known quadrature surface coils developed by others can be effectively used for imaging the spine. For example, when two separate coils, 90.degree. to each other are used; then, only one of the coils can be placed proximate to the spine while the other coil is kept away from the spine by the subject's body. The distance from the subject's body in effect makes a second coil irrelevant to the imaging process. Thus, until the invention of the previously mentioned quadrature surface coil provided by the inventor herein, none of the available planar pair resonators, counter-rotating quadrature current resonators or the quadrature surface coils were conducive to use in spinal imaging.
A problem with the previously mentioned quadrature surface coil invention is the previously mentioned problem, that the SNR signal is adversely related to the size of the coil. Thus, to make the surface coil sufficiently large to image the spine at one sitting results in a signal-to-noise ratio that is much too low for practical use. Therefore, when using the previously mentioned quadrature surface coils, it is necessary to take a plurality of acquisitions to image the entire spine. Apparatus for making such a plurality of acquisitions is described in U.S. Pat. No. 4,791,371, which issued on Dec. 13, 1988.
An array of surface coils for use in imaging a spine is taught in an abstract entitled "Simultaneous Multiple Surface Coil NMRI Imaging" by P. B. Roemer et al which appeared in the Book of Abstracts, Vol. 2 of the Society of Magnetic Resonance in Medicine at the Seventh Annual Meeting and Exhibition held on Aug. 20-26, 1988, in San Francisco. The same array is described in U.S. Pat. No. 4,825,162 which issued on Apr. 25, 1989. In that abstract and in the patent an array of coils is described wherein the adjacent coils overlap to prevent nearest neighbor interaction (cross coupling). The interaction between the next nearest neighbor is allegedly reduced by connection of each coil of the array to low input impedance (about 5 ohms) preamplifiers.
The problem with this solution is, among other things, the use of preamplifiers with low input impedance. Such low input impedance amplifiers are not standard and, therefore, are costlier. Also, the normal input impedance of the amplifiers is 50 ohms. By using amplifiers with input impedance of 50 ohms and less, a serious impedance matching problem is created.